1. Field of the Invention
This invention relates to an imaging process and element which rely upon a redox reaction between a cobalt(III)complex and a color developing agent, wherein the oxidized developing agent imagewise combines with a color coupler to form a dye image. A catalyst is provided for initiating the reaction.
2. State of the Prior Art
The use of redox reactions between cobalt(III)complexes and color developing agents, so that the oxidized developing agent combines with a dye coupler, is shown for example in Research Disclosure, Vol. 109, May, 1973, Publication No. 10911. In that teaching, the catalyst is silver. Although such systems have proven to be very satisfactory, they rely upon an initial silver image, and therefore upon a silver salt emulsion as the photographic element. The rapid increase in the cost of silver has necessitated the search for a less expensive substitute. A cheaper catalyst which performs as well as or better than silver is therefore a much sought-after improvement.
Other imaging systems based upon the reduction of a cobalt(III)complex have been introduced. Examples are shown in copending U.S. application Ser. No. 461,057, filed Apr. 15, 1974 by Anthony Adin and James C. Fleming, entitled "Transition Metal Photoreduction Systems and Processes", now abandoned and Ser. No. 461,171, filed on Apr. 15, 1974 by Albert T. Brault et al., entitled "Spectral Sensitization of Transition Metal Complexes", now abandoned commonly owned with the instant application. The imaging mechanism in those cases relies upon the formation of cobalt(II) and the decomposition of the ligands of the complex to form ammonia. Either of these reaction products in turn can be utilized to initiate an image, and a coupler may be incorporated to form a dye. For example, the aforesaid application Ser. No. 461,171 discloses thioamide functional groups, such as thiourea and thioacetamide which are incorporated along with the cobalt(III)complex, so that the reduced cobalt(II) combines with sulfide ions to form cobalt sulfide. The cobalt sulfide is either the final imaging means, or it may be secondary with the decomposed ligands forming the image in a separate, adjacent layer comprising a conventional diazonium salt, the coupler being in the adjacent layer. But in both these applications, there is no use of the cobalt sulfide image to catalyze a redox reaction.
Other patents which disclose in general the production of cobalt sulfide images or other metallic sulfide images are U.S. Pat. Nos. 1,880,449, issued to K. Hickman et al., and 2,084,420, issued to E. I. Weyde. These however do not teach the use of these images as catalysts.
Disclosures typical of the use, in an image-forming layer, of minute amounts of sulfide ion sources for sensitizing purposes and not as a primary imaging agent include the teachings of U.S. Pat. No. 1,623,499 issued to Shepard.